Possible Rhyhorn CD?

If Dialga is right around the corner, you could evolve when Dialga is announced. It's not clear to me that Dialga is next, but YMMV.

I wouldn't worry about having rhyperior for dialga, machamp is a much better counter to it due to how bad most ground moves are, even groudon can't compete with fighting types.

A possible CD for Rhperior could have its former signature move, Rock wrecker. This could be likely, as it is to rhyperior as the starters moves are for them. Conversely, it could also get Drill run, or perhaps earth power as a ground move. I'd say hold off on evolving rhydon now, ground wont be too effective against it compared to fighting types so you loose relatively little by not evolving it.

Unless perhaps it's a new dex entry, there's little point in evolving something if there's no need to do so. Even if Dialga arrives soon, I'm willing to bet you have other (likely better) counters for already, such as Groudon and Machamp.

After the news with Swinub community day no one can say for certain whether or not Rhyhorn will receive one. Of potential community day moves, Drill Run exists and would launch Rhyperior to the top of the DPS charts, even higher than Groudon against Pokemon with a double ground weakness like Heatran. It also has a signature rock type attack in the main series, Rock Wrecker that would have potential to make it the best rock type as well.

As mentioned before my post, you're better off using fighting types against Dialga than ground ones. Ground types need weather advantage to have an advantage over your standard fighting types, and even with it Rhyperior still has lower DPS than Breloom, Machamp, Hariyama, and potentially Blaziken depending on Dialga's move set.

Rhyperior's window of opportunity was against Heatran. That window has closed for now, so unless you need the dex entry I wouldn't evolve until ground types are in high demand again. If you need the dex entry I'd evolve a lesser one anyway.